With the spread of an ink jet recording system, there is a demand for speeding up of recording and improvement in quality of recorded images such as letters and photographs when recording is conducted on plain paper according to the ink jet recording system.
For example, it is required to record official documents, photographic images of digital cameras, and various kinds of information published in homepages on both sides of a plain paper sheet at high speed. It is also required to achieve such clear image quality as in a recorded image obtained through recording with a laser beam printer. It is further required to achieve a high image density when letter images are printed and to provide sharp letter images without loosing letter shapes even when small letter images are printed.
For recorded images such as those of color photographs and tables, it is also required to inhibit bleeding at a color boundary portion, which is caused by contact of a plurality of recording inks of different colors with each other.
In order to meet such requirements, there have been proposed an ink having low permeability and making combined use of a specified salt and a self-dispersion pigment (Japanese Patent No. 3862441) and an ink containing at least a specified self-dispersion pigment, a humectant and a glycerol ether compound (Japanese Patent Application Laid-Open No. 2006-274023). There has also been proposed an ink which contains organic ultrafine particles having an average particle size of 0.5 μm or less and internally and three-dimensionally crosslinked and provides a high-density image with suppressed bleeding on plain paper (Japanese Patent Application Laid-Open No. 2004-195706). There has further been proposed an ink in which the content of a permeability-imparting agent in the ink is more increased than such a content of the permeability-imparting agent that the lowering of a surface tension of the resulting ink composition attending on increase in the content thereof is stopped (Japanese Patent Application Laid-Open No. 2003-301129). There has further been proposed an ink containing a self-dispersion pigment including phosphonic acid as a surface functional group (WO 2007/053564).
The present invention relates to an ink jet recording ink, an ink jet image forming method and an ink jet recording apparatus, which are suitable for use in forming a recorded image on plain paper. Objects to be achieved are shown below.    1) An ink is fixed on plain paper in a short time.    2) A recorded image has a high density and is clear.    3) A recorded image is inhibited from causing bleeding between inks.    4) A recorded image is inhibited from causing strike-through.    5) Even when small letters are printed, the letters are sharp without loosing their shapes.    6) A recorded image has good water resistance and fixability.
Japanese Patent No. 3862441 and Japanese Patent Application Laid-Open No. 2006-274023 describe the inks making combined use of a self-dispersion pigment having —PO3(M)2 (M being a hydrogen atom, alkali metal, ammonium or organic ammonium) as a functional group and various kinds of organic carboxylic acid salts. According to these inks, sufficient effects are achieved on the above-described objects 2) to 5), but the objects 1) and 6) may not be achieved in some cases.
According to the inks described in Japanese Patent Application Laid-Open Nos. 2004-195706 and 2003-301129, water resistance and inhibition of bleeding between two color inks are improved to some extent. However, these inks are not such that all the above 6 objects are satisfied, and in particular, problems are left on the objects that the density of the recorded image is made high, the strike-through of the recorded image is inhibited, and the small letters are beautifully printed. WO 2007/053564 proposes such an ink that color developability is improved by using a self-dispersion pigment including phosphonic acid as a surface functional group; however, the color developability in high-speed printing is insufficient in some cases.
Accordingly, it is an object of the present invention to provide an ink jet recording ink, an ink jet image forming method and an ink jet recording apparatus, which sufficiently satisfy the above objects 1) to 6) at the same time.